Quick answer
Store cannabis flower at 58 to 62 percent relative humidity (RH) and 60 to 70 F in an airtight, opaque container kept in the dark. Below about 55 percent RH, trichomes dry out and terpenes fade. Above about 65 percent RH, mold and mildew can grow. A two-way humidity pack rated 58 or 62 percent plus a calibrated hygrometer keeps the jar inside that window.
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Target numbers
- Relative humidity: 58 to 62 percent
- Water activity: 0.55 to 0.65 aw
- Temperature: 60 to 70 F
- Light: none, so use opaque glass or a dark cabinet
- Air: sealed jar with little headspace
Moisture comes first because it drives everything else. Water activity is the free moisture that microbes can use, and ASTM D8197 sets 0.55 to 0.65 aw as the range for dry flower sold for consumption. That range matches what a sealed jar holds when it settles at 58 to 62 percent RH at room temperature.
Why the window is narrow
Two failure modes sit on either side of the range. Dry flower crumbles, burns hot, and loses the volatile terpenes that carry aroma. Wet flower invites botrytis and powdery mildew, and the damage is not always visible until the buds break open. Mold spores are already present on most plant material, and moisture is what lets them grow. Once mold sets in, the whole jar is a loss because spores spread through contact.
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What you need
- Airtight glass jar with a rubber gasket and clamp lid
- Digital hygrometer small enough to sit inside the jar
- Two-way humidity packs rated 58 or 62 percent
- Packs sized to the jar volume
- Dark drawer, cabinet, or box away from heat
How to set the humidity
- Calibrate the hygrometer with a salt test or a calibration kit before you trust its reading.
- Fill the jar no more than three quarters full so air can move around the buds.
- Drop in one humidity pack sized for the jar.
- Place the hygrometer on top of the flower and seal the lid.
- Wait 24 hours, then read the hygrometer.
- Add a second pack if the reading sits below 55 percent RH.
- Leave the lid off for 30 to 60 minutes if the reading sits above 65 percent RH, then seal the jar and check again after 12 hours.
- Repeat the check every few days for the first two weeks, then once a week after that.
Signs the humidity is off
- Too dry: buds crumble, stems snap, aroma fades, vapor feels harsh
- Too wet: buds feel spongy, jar walls fog, smell turns sharp or ammonia-like
- Mold: white fuzz, gray webbing, or dark patches near the stem
If you see mold, discard the whole jar. Do not try to dry the flower out or salvage part of it. Spores travel through the container.
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Daily stash vs long-term storage
Keep a small jar for the week and a larger jar in reserve. Opening the main jar lets in dry air and light, so each visit shifts the humidity. Long-term flower holds its numbers in a sealed jar at 58 percent RH with a pack, in a dark spot near 65 F. Refrigeration is not the right call for flower because the jar moves in and out of the cold, which pulls moisture to the surface. Freezing works for extracts and edibles, not for flower you plan to smoke.
Common mistakes
- Storing in plastic bags, which carry a static charge and let moisture pass
- Using a citrus peel or bread slice as a humidity source, which invites mold
- Packing jars tight with no headspace
- Leaving jars on a windowsill or near a radiator
- Trusting a hygrometer that has never been calibrated